Kotaku makes it sound like the devs are saying female characters are just somehow more complex technically than male characters, and they literally didn't have the technology before to create them. That's just obviously flat-out stupid.
What they're actually saying here is that their previous engine wasn't able to support custom characters, and so they never thought to try and make characters who would be representative of their full audience. When they made personalizing the character a focus, they realized they needed females to be an option, and that prompted them to add it.
It's still obviously sexist that it never even occurred to them there should maybe be a female in the game. There's no reason they couldn't have put one or two as options in the game before. But they never actually said it was impossible to do it. That's Kotaku's own not-very-generous interpretation.